1) Field of the Invention
The present invention relates to a process for producing a polyfluoroalkyl ester of unsaturated carboxylic acid, and more particularly to a process for producing a polyfluoroalkyl ester of unsaturated carboxylic acid useful as a starting monomer for polymers or as a starting compound for surfactants.
2) Related Art
Polyfluoroalkyl esters of unsaturated carboxylic acids, typically polyfluoroalkyl (meth)acrylate, have been hereto produced in an industrial scale as starting monomers for polymers with such characteristics as high transparency, chemical resistance, water- and oil-repellency, antifouling property, etc. due to specific nature of fluorine atoms contained in the molecules or as a starting compound for surfactants. However, due to now raised problems as to toxicity, accumulation, etc. of fluorotelomer, typically perfluorooctanoic acid (PFOA) and perfluorooctansulfonic acid (PFOS), it has been desired to substitute compounds with less toxicity and accumulation, but with equivalent characteristics.
International Journal of Cancer 78, issue 4, pages 491–495 (1998) reports that Rf groups having 2–5 or 16, 18 carbon atoms has a low inhibiting effect on intercellular information transmission performance (GJIC).
J. Fluorine Chemistry 5920 (2003), pages 1–20, discloses that Rf groups having carbon atoms around C3, which must be less than C8, can give a satisfactory surface activity, when formed into bifunctional compounds.
On the other hand, a process for producing carboxylic acid esters generally includes those based on dehydration condensation of carboxylic acid and alcohol in the presence of an acid catalyst or on chemical dehydration condensation in the presence of a dehydrating agent. Furthermore, a process based on dehydrohalogenation of carboxylic acid halide and alcohol is well known.
A process for producing a polyfluoroalkyl ester of unsaturated carboxylic acid includes those based on dehydration condensation of an unsaturated carboxylic acid and a polyfluoroalkanol in the presence of an acid catalyst such as p-toluenesulfonic acid, etc. with heating, using a non-polar solvent such as toluene, etc. as a water-azeotropic solvent, while distilling off the generated water, as disclosed in JP-A-63-210115 and JP-A-05-051417.
In these processes, however, polyfluoroalkanol is often inevitably removed to the outside of the reaction system due to the azeotropic boiling of polyfluoroalkanol and the dehydration solvent, and thus the expensive polyfluoroalkanol must be added in excess to the unsaturated carboxylic acid. Furthermore, the reaction must be carried out at elevated temperatures, and thus it is hard to suppress formation of polymers even in the presence of a polymerization inhibitor. Even in the industrial scale production, complicated apparatuses will be required for the removal of the generated water. Furthermore, in the recycle of used solvent for reuse, a large amount of expenditure will be required for the recycle for reuse, etc. as a problem due to the control of water content in the solvent or contamination of the solvent with impurities and by-products.
Still furthermore, in these processes, polyfluoroalkanol with a high acidity such as RfCH2OH has a very low reaction rate, resulting in occurrence of many side reactions, and thus it is very difficult to produce the desired product in high purity.
JP-A-62-92914 discloses a process based on dehydration condensation of itaconic acid and 1,1,1,3,3,3-hexafluoroisopropanol in an aprotonic polar solvent, using N,N′-dicyclohexylcarbodiimide (DCC) as a dehydrating agent, where the dehydrating agent DCC, though expensive, must be used in an equal amount to that of the alcohol, and also is impossible to recycle for the reuse, and thus generation of a large amount of wastes, etc. will inevitably occur as problems.
Other processes are also available, which are based on conversion of unsaturated carboxylic acids to carboxylic acid halides by various halogenating agents, followed by reaction with alcohols in the presence of a base, where an additional step of conversion to carboxylic acid halides as a starting material is required, that is, one more step is required in the process. Furthermore, the carboxylic acid halides are easy to hydrolize in the presence of water, resulting in strict water content control of the solvent to be used, etc. Hydrogen halides generated by the hydrolysis are highly corrosive, and thus the apparatuses must be made from expensive anti-corrosive materials, etc. Generation of a large amount of halides of the base used leads to a large amount of expenditure on the waste treatment.